[platform-netif] set link type to void (#4198)
[platform-netif] set link type to void (#4198)

This commit sets link type to void so that only one link-local
address will exist.
librasungirl/openthread,librasungirl/openthread,jwhui/openthread,srickardti/openthread,bukepo/openthread,jwhui/openthread,openthread/openthread,lanyuwen/openthread,jwhui/openthread,jwhui/openthread,srickardti/openthread,bukepo/openthread,abtink/openthread,abtink/openthread,openthread/openthread,openthread/openthread,chshu/openthread,chshu/openthread,lanyuwen/openthread,chshu/openthread,lanyuwen/openthread,openthread/openthread,abtink/openthread,srickardti/openthread,bukepo/openthread,chshu/openthread,chshu/openthread,bukepo/openthread,lanyuwen/openthread,chshu/openthread,srickardti/openthread,lanyuwen/openthread,librasungirl/openthread,abtink/openthread,librasungirl/openthread,lanyuwen/openthread